In this study we will use a model of ischemic bowel necrosis, or necrotizing enterocolitis (NEC) in rats and mice, by injecting PAF systemically. That PAF may be an important mediator for NEC is based on our previous findings: 1)PAF induces small bowel necrosis. 2)PAF mediates bowel injury induced by LPS, TNF or hypoxia. 3)PAF is rapidly produced after LPS injection, especially in the ileum. 4) PAF-R expression is very high in the small bowel. 5)Experimental NEC induced by hypoxia and formula feeding is prevented by PAF antagonist. 6) NEC patients have elevated serum PAF, and suppressed acetylhydrolase, the enzyme degrading PAF. We recently found that PAF at low dose (without gross bowel injury) also has profound effects on the small bowel, including inducing villus apoptosis and increasing mucosal permeability. Thus, Aim 1 is to examine the effect of low dose PAF on intestinal apoptosis and mucosal permeability (an early event before NEC occurs); and to study the respective mechanisms (roles of Fas/FasL, PMNs, lymphocytes, TNF, ROS, iNOS (inducible nitric oxide synthase), tyrosine kinase, eiconsanoids, tyrosine kinases, IFN- gamma and gut bacteria/LPS). Aim 2 is to test the hypothesis that intestinal necrosis occurs due to an imbalance between the protective cNOS (constitutive NOS) and the injurious iNOS ans xanthine oxidase (XO). This is based on our finding that PAF suppresses the former and stimulates the latter two. Aim 3 is based on our observations that (a) PAF up- regulates ileal PAF-R expression, an effect mediated via endogenous PAF and TNF, (b) PAF-receptor (PAF-R) localizes mainly in lamina propria (LP) eosinophils, (C) PAF permits LPS entry, which binds to LP eosinophils and (d)LPS/bacterial products are required for the development of NEC. We will examine the role of PAF-R, LP eosinophils and LPS in PAF induced LPS entry (a consequence of mucosal permeability increase), and its effect on the pathogenesis of bowel necrosis, release of mediators (e.g., PAF, LTC4,TNF) and consequent intestinal injury. These 3 closely related aims may lead to the elucidation of the pathophysiological function of PAF in the bowel, and the mechanisms of intestinal defense and injury.